Radiotransmitter



Apr. 17, 1923. 1,452,,Q64

V. BUSH RADIOTRANSMITTER Filed Nov 22 1918 :C A G Cane/77 in r/man/ J;(Wren/#7 an/nna Patented Apr. 17, 1923.

UNITED STATES earsur creme.

VANNEVAR BUSH, OF CHELSEA, MASSACHUSETTS, ASSIG-NOR TO AMERICAN RADIO &;RESEARCH CORPORATION, OF MEDFORD, MASSACHUSETTS, A CORPORATION OFDELAWARE.

RADIOTRANSMITTER.

Application filed November To all whom it may concern:

Be it known that I, .VANNEVAR BUsH, a citizen of the United States,residing at Chelsea, in the county of Suifolk and State ofMassachusetts, have invented certain new and useful Improvements inRadiotransmitters; and I do hereby declare the following to be a full,clear, and exact description of the invention such as will-enable othersskilled in the art to which it appertainsto make and use the same.

The present invention relates to radio transmitters having a primary orclosed circuit of the impact type in which a series of unidirectionalpulses are produced.

The best known types of commercial I transmitting apparatus embody anoscillating primary circuit having a spark gap and a secondary orantenna circuit tuned in resonance with the primary circuit and givingofl' group trains of waves. The eficient operation of this type oftransmitter is dependent directly upon an accurate tuning of the twocircuits, as otherwise the transference of a substantial amount ofenergy to the antenna circuit is precluded and the radiated energy fromthis latter circuit de creases rapidly. This accurate tuning of the twocircuits is diflicult to obtain under ordinary conditions, and thisdifficulty is more particularly evident in connection with ship,aeroplane, or portable installations where the antenna capacity changesrapidly.

The resurgence of current in a persistently oscillating primary circuit,moreover,tends to heat the gap and dissipate. the useful energy in thecircuit, and attempts have been made to overcome these objectionablefeatures by the'provision of a form of gap which quenches or damps theoscillation, and also by providing means for artificially dissipatingthe heat which is generated in the gap, However, the interposition ofthis quenched type of gap in the primary circuit renders even more acutethe difliculties due to a de-tuning of the two circuits which causes anirregular quenching action and a variation from the desired smooth tune.

The ideal condition for a primary circuit is that in which a single loopof unidirectional current passes through the gap at each discharge ofthe condenser, thus ensuring the utilization of the maximum amount orenergy and. permitting theefwient cow 22, 1918. Serial No. 263,757.

pling of two circuits which are not in resonance.

Repeated attempts have been made to secure a rectifying spark gap ofthis character which would cause the flow of a'unidirectional current inthe primary circuit, but these constructions have universally failed toattain the desired result, either because of the complicated andintricate nature of the constructions employed and the extremelydelicate adjustments required to cause the tapparatus to function in thedesired manner, or because of a misunderstanding of the principlesinvolved.

According to the present invention a flow of current is permitted whenthe gap initially breaks down and the condenser discharges, butthereafter the gap opens and a return flow of induced current isprevented. This object is accomplished by providing a primary circuitwith a gap of high sparking length as compared with the potentialemployed and connecting in circuit therewith impedance which renders thepotential gradient across the gap-highly nonuniform when this potentialis slowly raised, but which does not affect the normal uniform gradientwhen the potential across the gap is raised with sufiicient rapidity.This impedance, which may be due either to resistance, inductance,capacity or a combination' of these in the circuit, has a time constantwhen. considered in connection with the capacity of the gap of suchlength that the resurgence of current through the gap at radio period isprevented and a pulsating unidirectional current produced. Thispulsation of current in the primary circuit produces true impactexcitation as distinguished from the operation of the quenched gap whichmerely serves to damp the oscillations in the primary circuit. In thepresent invention the primary and second ary circuits are closelycoupled and detuned. that is. the two circuits do not have the samenatural period of frequency. The free wave lengths of the primary andsecondary circuits, however, may have a definite numerical relation sothat the pulses in the primary circuit produce the maximum possibleeffect upon the secondary circuit.

A further feature of the invention consists in producing impactvexcitation in the primary circuit of a radi transmitter by firstbringing a portion of the unit gaps of a series spark gap to the samepotential and thereafter repeating the operation until all of the gapsof the series have been brought to v the same potential, breaking downthe entire gap.

In the accompanying drawings illustrating the invention, Figure 1illustrates diagrammatically a circuit provided with a resistance shuntacrossa portion or the unit gaps of a series gap; Fig. 2 illustrates agap employing such a shunt in which the capacity between the plates hasbeen increased by the connection of an auxiliary condenser; Fig. 3 showsa circuit having inductance shunted across a portion of the unit oraseries gap; Fig. 4- represcnts diagrammatically the current which flowsin the primary circuit due to this form of gap upon each'discharge ofthe condenser; and Fig. 5

represents the character of current flow, in

the antenna circuit which is induced by each loop of current in theprimary circuit. l The illustrated embodiment of the invention is shownin connection with a transmitting set having a generator D, a condenserC, the primary P of an oscillation trans former, and the secondary S ofthe oscillation transformer. The primary circuit is. provided'with aseries gap G which, as shown in Fig. 1, may consist of eight unit gaps,and is preferably a gap which rapidly deionizes after the discharge ofthe spark. The potential supplied by the generator l) is insufficienttobreak down all of the unit gaps comprising the series, but is sufficientto break down part of these gaps A resistance r is accordingly shownconnecting; middle plate of the series gap with one ,end, and inconsequence the plates 1- and 5 are brought to approximately the samepotential almost immediately after potential. is applied to the circuit,the length of time required for this depending upon the value of theresistance r and also upon the value of capacity between the difi'erentplates of the quenched gap. When the plates 1 to 5 Teach thesamepotential the full. potential the generator D is applied across thereinaining four gaps 5 to 9, and since this pot'eiitial is sufficient tobreak down four gaps in the series a spark will then pass from plates 5to 9. Thereupon the full potential is brought to bear across plates 1.to 5 by tin tue oi the fact that plates 5 to 9 have now been reduced tothe same potential by the passage of the spark and the remainder of thegap willthen break down. thus causing: the passage of a spark across allof the gaps oi the series. When this occurs the condenser C dischargesthrough the primanv P, of the oscillation transformer and thetransmitter operates.

The flpw of the current thro this type of gap" is illustrateduiagrarniaatically in secondary circuit.

Fig. 4 andas indicated is confined to substantially a single loop ofunidirectional current. This action creates a series of unidirectionalpulses in the primary circuit and permits close coupling with thetransference of a maximum amount ofenergy to the In order to accomplishthis new and important result it is necessary, as stated previously thatthe time constant of the resistance 1 in connection with the capacity ofthe gap plates be small as compared with the tone period and largecompared with the radio period of the circuit. During the comparativelylong time which elapses :tor the charging'o't'the condenser U at audiofrequency there will be suflicient leakage through the resistance r toproperly distribute the potential gradient across the gap so thatbreak-'dowu'will occur. After the break-down has occurred, however, andV the condenser C discharges through the primary P there is notsullicicnt time during the radio period for a similar break-down to takeplace in the reverse directioinowing to the fact that in this shortinterval there is an inappreciable tlowoft current throng the shuntedmpedance which is entirely insuflicient to charge the capacity and detil ill]

wise cause a reverse'fio'w oi current encoun- I ters the full dielectricstrength of the gap at uniform gradient. Since this reverse potentialcannot possibly exceed the potential which was on the primary condenserjust before break-down, tl1e gap will hence not break down intherever'se direction, and the primary current will bepulsating andunidirectional.

This action should not. be confused with that which would occuriftheshunt 2' were chosen at random and without reference to the gapcapacity, gap of negl' ible capacity between plates isusecl, or it theshunt r is chosen too si'nallwith'reference to the gap capacity, thegradient across the gap will be non-uniform even when the potentialacross the gap rises at a rate determined by the radiot' equency. Thegap will then break down again in the reverse direction ai tcr theinitial pulse. and the t' marycurrent will be oscillstory and no Ieating: The effect of introducii of this latter sort, which is notcorrectlated to the gap capacity and the audio and radio frequenciesusechvcill hence be "1 ply to cause somewhat more rn iiid quench iii-iwithout essentially changing the urethral operation of the apparatus. i

The following illustrates one method cl estimating the resistance r forp roperopera lif of capacity of plates 1 to 5 in Fig. 1, and if .thetone period is 10* second'corresponding to a tone ire uenc otonethousand c cles then r must be designed so that the product middle plateshall follow the potential of the end plat-e with tone frequencies butat radio frequencies this action does not take place, renderingimpossible a break-clown of the gap after the initial loop of primarycurrent has passed therethrough and thus securing an impact excitation.

Figure 2 illustrates an arrangement of gap in which the capacity cotgaps 1 to 5 is increased bygthe condenser of capacity C connected inparallel therewith. The time constant of this gap is now 1" (0 C). Thisarrangement is sometimes advantageous since it allows a smaller value ofresistance r to be used.

To illustrate this effect, suppose that the capacity C of 9 X 10* faradsis added to the gap just considered. Then the total capacity (0 C)between 1 and 5 is 10- farads. If the time constant 1" (c O) is to haveas before the value 10' second 1" should be given the value 10,000 ohms.It will not be necessary that 1 should have exactly this value, butmerely that it should be approximately this value.

Figure 3 illustrates a further modification of a primarycircuit in whichimpedance 2 is connected in shunt across some of the unit gaps ofthespark discharger, the time constant of the impedance 2 in connectionwith the capacity of the unshunted gap plates being so proportioned thata return flow of current may not pass through the gap at radiofrequencies.

The drawings illustrate a tone circuit T which is included in theprimary circuit when a direct current generator is employed and operatesin the usual manner to produce a tone of any desired pitch in theemitted note.

It will be obvious to those skilled in the art that this type of gap maybe employed to equal advantage in radio sets transmitting with asustained oscillation from the antenna circuit as well as with setswhich signal by the radiation of train group waves from the antennacircuit. When using the rectifying gap in the first instance, the frequency of discharge is considerably above audible limits and may bear apredetermined relationship to the natural period of oscillation of theantenna circuit. How

ever, whether or not the discharges in the exciting circuit-do bear acertain definite numerical relation to the time period of the antennacircuit it is a characteristic of the unidirectional pulses set upin-the primary circuit that they invariably boost'the oscillations inthe antenna circuit, the oscillations in the antenna circuit apparentlytriggering off these pulses at the proper time to invariably boost andnot retard the oscillations. l Vhen this type of gap is employed inconnection with transmitters which radiate waves of train group hequency from the antenna circuit it is necessary to so adjust theconstants of the exciting and antenna circuits that they are not in.resonance with one another and have difterent natural periods offrequency.

What is claimed is:

1.. A spark gap for radio transmitting systems comprising a series ofunit gaps and means connected therewith and having in conjunction withthe capacity of the gap a time constant which is small compared with theaudio period of the transmitting system and large compared with radioperiod of the transmitting system to cause the gap to be broken down bya potential which rises at audio frequencies but to be unaffected by thesame potential rising across the gap at radio frequency.

2. A radio transmitter comprising a spark gap and impedance shuntedaround the gap and having in conjunction with the capacity of the gap 2.time constant which is small compared with the audio period of thetransmitting circuit and large compared with the radio period of thecircuit.

3. A radio transmitter, comprising a primary circuit including agenerator, a primary condenser; a gap of high sparking length comparedwith the potential of the circuit, an impedance connected with the gapand having in connection with the capacity of the gap a time constantwhich is small compared to the audio period of the circuit and largecompared to the radio period of the circuit to cause the flow of aunidirectional current in the circuit, and an antenna circuit coupled tothe primary circuit.

4:. A radio transmitter, comprising a primary circuit which includes aseries spark gap of high sparking length compared with the potential ofthe circuit, and impedance shunted across a portion of the unit gaps ofthe series gap to render the potential across the gap highlynon-uniform, the impedance having in conjunction with the capacity ofthe gap a time constant which is small compared with the audio period ofthe circuit and large compared with the radio period.

5. A radio transmitter, comprising a primary circuit which includesagenerator, a

primary condenser, a spark gap of high sparking length compared With thepotential of the circuit and consisting of a plurality of unit gaps, andimpedance shunted across a portion of the unit gaps tocause the initialbreaking down of partof the unitgaps of theseries gap and a finalbreaking down of all of the gaps in the series gap, the impedance havingin conjunction with the capacity of the gap a time constant whichissmall 10 compared with the audio period of the circuit and largecompared With the radioperiod.

VANNEVAR BUSH.

